1. Field of the Invention
The present invention relates to a magnetic disk drive and a head positioning control method for magnetic disk drives. More particularly, this invention is concerned with a head positioning control system and control method for magnetic disk drives which enables fast seek and gives control readily even when servo-control circular trajectories defined with servo control information recorded on the surfaces of disks do not match with a rotational circular trajectory because of eccentricities of the disks.
2. Description of the Related Art
In recent years, there has been the trend toward higher recording densities for the purpose of increasing the storage capacity of magnetic disk drives. A faster access speed is also requested. Various methods have been proposed to increase the storage capacity of a magnetic disk drive and improve the access speed thereof.
A magnetic disk drive (hereinafter a disk drive) stores data as magnetic patterns on magnetic disks (hereinafter disks) using magnetic heads (hereinafter heads), and reads data by detecting an electrical signal in a head caused by a recorded magnetic pattern. For specifying a position at or from which data is stored or read, magnetic guides referred to as tracks are formed concentrically with the center of rotation of each disk as a center. A position in a radial direction is identified by detecting a track. Each track is divided in a circumferential direction into a plurality of segments referred to as sectors. Sector numbers indicating circumferential position thereof are magnetically recorded on each disk. Data storage is carried out in units of a sector.
In a disk drive in which a plurality of disks are fitted on the same axis of rotation and heads are placed on the record surfaces of the disks, a control method referred to as a servo-control surface servo-control method is adopted. This control method is, for example, such that: the heads are interlocked with one another and moved; one of the disk surfaces is regarded as a servo-control surface, and information concerning tracks and sectors is recorded on the servo-control surface; and the other disk surfaces are regarded as data surfaces on which data alone is recorded. However, in the servo-control surface servo control method, the displacement of a head on a data surface relative to a head on a servo-control surface leads to a positional error of the head relative to the position of written data. The positional error cannot be reduced very much because of a temperature change or vibration of a head moving mechanism. It is therefore difficult to sufficiently narrow track width and thus improve recording density. In recent years, data has been recorded on all disk surfaces. Information concerning a sector and information concerning a track are recorded at the start of each sector on each disk surface, and detected in order to control the position of a head. Moreover, signals indicating sectors are magnetically recorded on a disk. The magnetic data concerning sectors is read in order to identify a sector.
In an existing disk drive, disks are fitted on the axis of rotation of a spindle motor, magnetic heads are attached to an actuator, and then the disks and magnetic heads are assembled within a housing. Thereafter, the housing is mounted on a servo track writer (STW), so that servo control information (including track numbers and sector numbers) can be written while the actuator inside the disk drive is being moved by means of an actuator installed outside the disk drive and controlled to be positioned highly precisely. After this processing is completed, it becomes possible to move the actuator for writing or reading data. Tracks are therefore concentric trajectories with the center of rotation as a center. The same applies to data surface servo control and servo-control surface servo control. However, in reality, since the heads or spindle motor vibrates while data is being written along a track, the track is not a complete circle but includes vibrations relative to a circular trajectory. For improving the recording density of a disk drive, it is required to narrow the track width and increase the number of tracks that can be formed on a disk. However, if a track vibrates, there is the fear that adjoining tracks may interfere with each other. The track width cannot therefore be narrowed so drastically. This obstructs improvement of a recording density.
The present inventor has determined that a recording density might be improved by forming tracks accurately on disks using an external apparatus and then assembling the disks by fitting them on an axis of rotation. As long as tracks are merely formed, it is unnecessary to take account of an access speed or the like. Tracks can be formed more accurately using a dedicated head. Consequently, track width can be reduced and recording density can be raised. Currently, for writing servo control information, a head is attached to a swinging suspension and used for the writing. In an external apparatus, the head attached to the swinging suspension is regarded as a head assembly that hardly swings and is used for writing servo control information. Thus, the number of tracks on each disk is increased. Consequently, high-density recording of data can be achieved. However, when disks having tracks formed are fitted on the axis of rotation of a spindle motor, even if the fitting precision is improved, it is unavoidable that the centers of the tracks are not aligned with the center of rotation and the tracks become eccentric to some extent. It is conceivable that adjustment is carried out in order to alleviate eccentricities. However, very delicate adjustment is needed. It is very hard to carry out adjustment until sufficient precision is attained. Even if such adjustment is possible, there arises a problem that too much man power is required for the adjustment and this leads to an increase in cost.
According to the data surface servo control method for a plurality of disk surfaces, only a head engaged in writing or reading data should be controlled for tracking. Feedback is carried out so that a head to be controlled can track the center of a track. For changing a head to be controlled to another, a standby state is retained until the new head tracks a target track. After the head is controlled to track the center of the target track, data writing or reading is carried out. This poses a problem that after heads are changed, a long access time ensues.